Critical Adsorption of a Pure Fluid on a Graphite Substrate

The surface excess amount $\Gamma$ of S${\mathrm{F}}_6$ on graphitized carbon (measured by a new volumetric technique) rises sharply on approach to the critical point along an isochoric path from the one-phase region of the fluid. For a limited range of reduced temperatures ($0.02>t>\mathrm{}0.001\mathrm{}$) $\Gamma$ varies as a power law in $t$. Our data yield an exponent $x=0.5\mathrm{}$, whereas the Fisher-de Gennes scaling theory predicts $x=\nu -\beta =0.3\mathrm{}$. Close to criticality confined-geometry effects become important.